Field of invention
[0001] The invention relates to a polymer composition suitable for producing a layer of
a cable, to a cable comprising said polymer composition which is preferably crosslinkable,
to a process for preparing at least one layer of a wire or cable by using said polymer
composition and preferably by crosslinking said wire or cable layer.
Background art
[0002] A typical electric power cable generally comprises a conductor that is surrounded
by several layers of polymeric materials including an inner semiconducting layer,
followed by an insulating layer, and then an outer semiconducting layer. These layers
can be crosslinked as well known in the field. To these layers, one or more further
auxiliary layer(s) may be added, such as a screen and/or a jacketing layer(s) as the
outer layer. The layers of the cable are based on different types of polymers. E.g.
low density polyethylene, crosslinked by adding peroxide compounds, is a typical cable
insulation material.
[0003] Polyolefins, particularly polyethylene is generally used as an electrical insulation
material as it has good dielectric properties, especially high breakdown strength.
Such insulated cables have, however, a drawback that they suffer from shortened service
life when installed and operated in an environment where the polymer is exposed to
water, e.g. under ground or at locations of high humidity. The reason is that polyolefins
tend to form bush-shaped defects, so-called water trees, when exposed to water under
the action of electrical fields. Such defects may be due to inhomogeneities, for instance
microcavities and impurities, such as contaminants, occurring in the layer material
and they can lead to lower breakdown strength and possibly electric failure when in
use.
[0004] The appearance of water tree structures are manifold. In principle, it is possible
to differentiate between two types:
"Vented trees" which have their starting point on the surface of the semiconductive
layer(s) and
"Bow-tie trees" which are initiated within the insulation material often starting
from a defect or a contaminant.
[0005] Water treeing is a phenomenon that has been studied carefully since the 1970's. Many
solutions have been proposed for increasing the resistance of insulating materials
to degradation by water-treeing. One solution involves the addition of polyethylene
glycol, as water-tree growth inhibitor to a low density polyethylene such as described
in
US 4,305,849 and
US 4,812,505. Furthermore, the invention
WO 99/31675 discloses a combination of specific glycerol fatty acid esters and polyethylene glycols
as additives to polyethylene for improving water-tree resistance. Another solution
is presented in
WO 85/05216 which describes copolymer blends.
[0006] Further,
EP 1 695 992 describes at least one ether and/or ester group containing additive that is combined
with an unsaturated polyolefin in order to provide a polyolefin composition with enhanced
crosslinking properties and improved water-tree resistance.
[0007] Accordingly, there is a continuous need to find alternative solutions with advantageous
water tree resistance properties to overcome the prior art problems.
Description of the invention
[0008] The invention provides a polymer composition comprising
- a polyolefin (A) and
- an antioxidant selected from a phenol which bears two substituents both containing
a sulphur atom and optionally further substituent(s), and
wherein the polymer composition comprises vinyl groups/1000 carbon atoms in an amount
of 0.15/1000 carbon atoms or more, when determined according to "double bond content"
described below under Determination methods.
[0009] Surprisingly, the unsaturation, i.e. the vinyl groups, present in the polymer composition
combined with the specific antioxidant compound as defined above has improved water
tree resistance (WTR) properties compared to WTR properties of a same polymer composition
but without the presence of the above defined amount of vinyl groups, when measured
according to WTR method as described below under the determination methods.
[0010] The polymer composition of the invention is referred herein below also shortly as
"polymer composition". The polymer components thereof as defined above are also shortly
referred herein as "polyolefin (A)" and, respectively, "antioxidant".
[0011] In general, "vinyl group" means herein CH
2=CH- moiety.
[0012] The polymer composition preferably comprises vinyl groups/ 1000 carbon atoms in an
amount of 0.15/1000 carbon atoms or more, preferably in an amount of 0.20/1000 carbon
atoms or more, preferably in an amount of 0.25/1000 carbon atoms or more, more preferably
in an amount of 0.30/1000 carbon atoms or more. More preferably, the amount of vinyl
groups/1000 carbon atoms present in the polymer composition is less than 4.0/1000
carbon atoms, more preferably less than 3.0/1000 carbon atoms.
[0013] The polymer composition comprises vinyl groups as carbon-carbon double bonds, which
vinyl groups originate preferably from
- i) a polyunsaturated (co)monomer,
- ii) a chain transfer agent,
- iii) an unsaturated low molecular weight compound which is e.g. a compound known as
a crosslinking booster or as a scorch retarder, or
- iv) any mixture of (i) to (iii).
[0014] The total amount of vinyl groups means herein the sum of the vinyl groups present
in the vinyl-group sources, if many sources. It is evident that a characteristic model
compound for calibration is used for each chosen source to enable the quantitative
infrared (FTIR) determination. The total amount of vinyl-groups means herein double
bonds determined from the source(s) that are known and deliberately added to contribute
to the unsaturation.
[0015] It is preferred that the polymer composition comprises a polyolefin (A) which is
unsaturated and comprises vinyl groups in a amount as defined above or below or in
claims.
[0016] The polymer composition preferably further comprises one or more crosslinking agent(s).
The preferred crosslinking agent is a free radical generating agent, preferably a
free radical generating agent containing -O-O- bond or -N=N-bond. More preferably,
the crosslinking agent is a peroxide. As non-limiting examples of suitable organic
peroxides, di-tert-amylperoxide, 2,5-di(tert-butylperoxy)-2,5-dimethyl-3-hexyne, 2,5-di(tert-butylperoxy)-2,5-dimethylhexane,
tert-butylcumylperoxide, di(tert-butyl)peroxide, dicumylperoxide, butyl-4,4-bis(tert-butylperoxy)-valerate,
1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, tert-butylperoxybenzoate, dibenzoylperoxide,
bis(tert butylperoxyisopropyl)benzene, 2,5-dimethyl-2,5-di(benzoylperoxy)hexane, 1,1-di(tert-butylperoxy)cyclohexane,
1,1-di(tert amylperoxy)cyclohexane, or any mixtures thereof, can be mentioned. Preferably,
the peroxideis selected from 2,5-di(tert-butylperoxy)-2,5-dimethylhexane, di(tert-butylperoxyisopropyl)benzene,
dicumylperoxide, tert-butylcumylperoxide, di(tert-butyl)peroxide, or mixtures thereof.
Most preferably, the peroxide is dicumylperoxide.
[0017] Preferably, the polymer composition comprises the crosslinking agent, preferably
peroxide, in an amount of less than 10 wt%, less than 6 wt%, more preferably of less
than 5 wt%, less than 3.5 wt%, even more preferably from 0.1 wt% to 3 wt%, and most
preferably from 0.2 wt% to 2.6 wt%, based on the total weight of the polymer composition.
[0018] Accordingly, the polymer composition is preferably crosslinkable and optionally,
and preferably, is in crosslinked form at the end use thereof. "Crosslinkable" is
a well known expression and means that the Polyolefin Composition can be crosslinked,
e.g. via radical formation, to form bridges i.a. amongst the polymer chains.
[0019] Any double bond measurements are carried out prior to optional crosslinking. Moreover,
it is evident that the below given polyolefin (A) descriptions apply to the polyolefin
prior optional crosslinking.
[0020] The polymer composition of the invention may naturally contain further components,
such as further polymer component(s), additive(s) or any mixtures thereof.
[0021] As an example only such additives include one or more of antioxidants, stabilisers,
processing aids, scorch retardants, crosslinking boosters or water tree retardants,
or any mixtures thereof. As antioxidant, sterically hindered or semi-hindered phenols,
optionally substituted with functional group(s), aromatic amines, aliphatic sterically
hindered amines, organic phosphates, thio compounds, and mixtures thereof, can be
mentioned. Typical cross-linking boosters may include compounds having a vinyl or
an allyl group, e.g. triallylcyanurate, triallylisocyanurate, and di-, tri- or tetraacrylates.
As preferable scorch retardants, e.g. unsaturated dimers of aromatic alphamethyl alkenyl
monomers, such as 2,4-di-phenyl-4-methyl-1-pentene, can be mentioned. Such scorch
retardants can also act as crosslinking boosters. As further additives, flame retardant
additives, acid scavengers, fillers, such as carbon black, and voltage stabilizers
can be mentioned. All the above mentioned additives are well known in polymer field.
Such compositions are very useful for wire and cable applications, such as for cables
of the invention discussed below.
[0022] The polymer composition of the invention comprises typically at least 50 wt%, preferably
at least 75 wt%, preferably from 80 to 100 wt% and more preferably from 85 to 100
wt%, of the polyolefin (A) based on the total weight of the polymer component(s) present
in the polymer composition. The preferred polymer composition consists of polyolefin
(A) as the only polymer component. The expression means that the polymer composition
does not contain further polymer components, but the polyolefin (A) as the sole polymer
component. However, it is to be understood herein that the antioxidant of the polymer
composition or further components other than polymer components, such as additives
which may optionally be added in a mixture with a carrier polymer, i.e. in so called
master batch. In such cases the carrier polymer of the master batch is not calculated
to the amount of the polymer components, but to the total amount of the polymer composition.
[0023] The following preferable embodiments, properties and subgroups of the polyolefin
(A) and the antioxidant components suitable for the polymer composition are independently
generalisable so that they can be used in any order or combination to further define
the preferable embodiments of the polymer composition and the cable produced using
the polymer composition as described below.
[0024] As to antioxidant, e.g. any conventional or commercially available phenolic antioxidant
with the substitution as defined above or below are suitable for the polymer composition.
The antioxidant is preferably bis[(C1-C12)alkylthio(C1-C12)alkyl]phenol which optionally
bears further substituent(s).
[0025] In a preferable embodiment of the polymer composition the antioxidant is 2,4-bis(octylthiomethyl)-6-methylphenol
(CAS number 110553-27-0) which is commercially available, e.g. sold as Irgastab® Cable
KV10 product, supplied by Ciba, and which has a following structure:
[0026] It is preferred that the polymer composition comprises the antioxidant of the invention
as the only antioxidant. This means that no compounds conventionally known as W&C
antioxidant are present.
[0027] As to polyolefin (A), a suitable polyolefin can be any conventional polyolefin, in
particular which can be used in a layer of a cable, preferably of a power cable. Such
suitable polyolefins are, for example, well known and can be commercially available
or can be prepared according to or analogously to known polymerization processes described
in the chemical literature. Where herein it is referred to a "polymer", e.g. polyolefin,
this is intended to mean both a homo- and copolymer, e.g. an olefin homo- or copolymer,
such as an ethylene homo- and copolymer. The polyolefin copolymer may contain one
or more comonomer(s). As well known, the term "comonomer" refers to copolymerisable
comonomer units.
[0028] In the preferred embodiment of the polymer composition as mentioned above, the polyolefin
(A) is unsaturated and comprises vinyl groups/1000 carbon atoms in an amount of 0.15/1000
carbon atoms or more, preferably in an amount of 0.20/1000 carbon atoms or more, preferably
in an amount of 0.25/1000 carbon atoms or more, more preferably in an amount of 0.30/1000
carbon atoms or more, and preferably the amount of vinyl groups is less than 4.0/1000
carbon atoms, more preferably is less than 3.0/1000 carbon atoms.
[0029] As also well known, "unsaturated polyolefin (A)" means herein both 1) a homopolymer
or a copolymer, wherein the unsaturation is provided by a chain transfer agent or
by adjusting the process conditions, or both, and 2) a copolymer, wherein the unsaturation
is provided at least by polymerizing a monomer together with at least a polyunsaturated
comonomer and optionally by other means, such as by adjusting the polymer conditions
or by a chain transfer agent.
[0030] In this preferred embodiment the unsaturated polyolefin (A) is preferably an unsaturated
polyethylene. In general, for polyethylene, ethylene will form the major monomer content
present in any polyethylene polymer.
[0031] Where the unsaturated polyolefin (A) is an unsaturated copolymer of ethylene with
at least one comonomer, then suitable comonomer(s) are selected from polyunsaturated
comonomer(s), and further comonomer(s), such as non-polar comonomer(s) other than
polyunsaturated comonomer(s) or polar comonomer(s), or any mixtures thereof. The polyunsaturated
comonomers and further comonomers, i.e. non-polar comonomers other than polyunsaturated
comonomers and polar comonomers are described below in relation to polyethylene produced
in a high pressure process.
[0032] If the preferred unsaturated polyolefin (A) is a copolymer, it preferably comprises
0.001 to 50 wt.-%, more preferably 0.05 to 40 wt.-%, still more preferably less than
35 wt.-%, still more preferably less than 30 wt.-%, more preferably less than 25 wt.-%,
of one or more comonomer(s).
[0033] Preferably, the unsaturated polyolefin (A) is an unsaturated polyethylene produced
in the presence of an olefin polymerisation catalyst or a polyethylene produced in
a high pressure process.
[0034] "Olefin polymerisation catalyst" means herein preferably a conventional coordination
catalyst. It is preferably selected from a Ziegler-Natta catalyst, single site catalyst
which term comprises a metallocene and a non-metallocene catalyst, or a chromium catalyst,
or any mixture thereof. The terms have a well known meaning.
[0035] In general, the polyethylene polymerised in the presence of an olefin polymerisation
catalyst is also often called as "low pressure polyethylene" to distinguish it clearly
from polyethylene produced under high pressure. Both expressions are well known in
the polyolefin field. Low pressure polyethylene can be produced in polymerisation
process operating i.a. in bulk, slurry, solution, or gas phase conditions or in any
combinations thereof. The olefin polymerisation catalyst is typically a coordination
catalyst. More preferably, the unsaturated polyolefin (A) is selected from an unsaturated
homopolymer or a unsaturated copolymer of ethylene produced in the presence of a coordination
catalyst or produced in a high pressure polymerisation process.
[0036] In said preferred embodiment of the polymer composition it is more preferred that
the unsaturated polyolefin (A) is an unsaturated polyethylene produced in a high pressure
polymerisation process, preferably by radical polymerisation in the presence of an
initiator(s). More preferably the unsaturated polyolefin (A) is an unsaturated low
density polyethylene (LDPE). It is to be noted that a polyethylene produced in a high
pressure (HP) is referred herein generally as LDPE which term has a well known meaning
in the polymer field. Although the term LDPE is an abbreviation for low density polyethylene,
the term is understood not to limit the density range, but covers the LDPE-like HP
polyethylenes with low, medium and higher densities. The term LDPE describes and distinguishes
only the nature of HP polyethylene with typical features, such as high branching degree,
compared to the PE produced in the presence of an olefin polymerisation catalyst.
[0037] This more preferred unsaturated LDPE polymer can be an unsaturated low density homopolymer
of ethylene (referred herein as unsaturated LDPE homopolymer) or an unsaturated low
density copolymer of ethylene with one or more comonomer(s) (referred herein as unsaturated
LDPE copolymer).
[0038] If the unsaturated LDPE is an unsaturated LDPE homopolymer, then the unsaturation
can be provided e.g. by a chain transfer agent (CTA), such as propylene, and/or by
polymerization conditions as mentioned above. If the unsaturated LDPE polymer is an
unsaturated LDPE copolymer, then the unsaturation can be provided by one or more of
the following means: by a chain transfer agent (CTA), by one or more polyunsaturated
comonomer(s) or by polymerisation conditions. It is well known that selected polymerisation
conditions such as peak temperatures and pressure, can have an influence on the unsaturation
level. In case of an unsaturated LDPE copolymer, it is preferably an unsaturated LDPE
copolymer of ethylene with at least one polyunsaturated comonomer, and optionally
with other comonomer(s), such as polar comonomer(s) as described below.
[0039] In said preferred embodiment of the polymer composition, it is most preferred that
the unsaturated polyolefin (A) is an unsaturated LDPE copolymer of ethylene with at
least a polyunsaturated comonomer(s). The polyunsaturated comonomer(s) suitable for
the unsaturated polyolefin preferably consist of a straight carbon chain with at least
8 carbon atoms and at least 4 carbons between the non-conjugated double bonds, of
which at least one is terminal. More preferably, said polyunsaturated comonomer is
a diene, preferably a diene which comprises at least eight carbon atoms, the first
carbon-carbon double bond being terminal and the second carbon-carbon double bond
being non-conjugated to the first one. Preferred dienes are selected from C
8 to C
14 non-conjugated dienes or mixtures thereof, more preferably selected from 1,7-octadiene,
1,9-decadiene, 1,11-dodecadiene, 1,13-tetradecadiene, 7-methyl-1,6-octadiene, 9-methyl-1,8-decadiene,
or mixtures thereof. Even more preferably, the diene is selected from 1,7-octadiene,
1,9-decadiene, 1,11-dodecadiene, 1,13-tetradecadiene, or any mixture thereof, however,
without limiting to above dienes.
[0040] In said preferred embodiment of the polymer composition, where the unsaturated polyolefin
(A) is the unsaturated LDPE copolymer of ethylene with at least a polyunsaturated
comonomer, the vinyl groups present in the polymer composition originate at least
partly from the polyunsaturated comonomer. More preferably, the total amount of said
vinyl groups which originate from the polyunsaturated comonomer is, in the given preference
order, of 0.15/1000 carbon atoms or more, preferably of 0.20/1000 carbon atoms or
more, preferably of 0.25/1000 carbon atoms or more, more preferably of 0.30/1000 carbon
atoms or more, and preferably less than 4.0/1000 carbon atoms, more preferably less
than 3.0/1000 carbon atoms.
[0041] Naturally the most preferred unsaturated LDPE polymer may comprise further carbon-carbon
double bonds, such as those which typically originate from vinylidene groups and
trans-vinylene groups, if present.
[0042] As already mentioned, the most preferred unsaturated LDPE copolymer of ethylene with
at least a polyunsaturated comonomer may optionally comprise "further" comonomer(s).
Such "further" comonomers are preferably selected from the polar comonomer(s), non-polar
comonomer(s) other than polyunsaturated comonomers or from a mixture of the polar
comonomer(s) and such non-polar comonomer(s).
[0043] As a polar comonomer for the unsaturated LDPE copolymer as said most preferred unsaturated
polyolefin (A), if present, then comonomer(s) containing hydroxyl group(s), alkoxy
group(s), carbonyl group(s), carboxyl group(s), ether group(s) or ester group(s),
or a mixture thereof, can be used. More preferably, comonomer(s) containing carboxyl
and/or ester group(s) can be used as said polar comonomer. Still more preferably,
if present, the polar comonomer(s) of LDPE copolymer is selected from the groups of
acrylate(s), methacrylate(s) or acetate(s), or any mixtures thereof. If present in
said preferred unsaturated LDPE copolymer, the polar comonomer(s) is preferably selected
from the group of alkyl acrylates, alkyl methacrylates or vinyl acetate, or a mixture
thereof. Further preferably, said optional polar comonomers can be selected from C
1- to C
6-alkyl acrylates, C
1- to C
6-alkyl methacrylates or vinyl acetate. The optionally polar unsaturated LDPE copolymer
is most preferably a copolymer of ethylene with C
1- to C
4-alkyl acrylate, such as methyl, ethyl, propyl or butyl acrylate, or vinyl acetate,
or any mixture thereof.
[0044] As the non-polar comonomer(s) other than polyunsaturated comonomers for the unsaturated
LDPE copolymer as said most preferred polyolefin (A), if present, then comonomer(s)
other than the above defined polar comonomers can be used. Preferably, such non-polar
comonomers are other than comonomer(s) containing hydroxyl group(s), alkoxy group(s),
carbonyl group(s), carboxyl group(s), ether group(s) or ester group(s). If present,
then preferable non-polar comonomer(s) can be selected from monounsaturated (= one
double bond) comonomer(s), preferably olefins, preferably alpha-olefins, more preferably
C
3 to C
10 alpha-olefins, such as propylene, 1-butene, 1-hexene, 4-methyl-1-penten styrene,
1-octene, 1-nonene; a silane group containing comonomer(s); or any mixtures thereof.
[0045] It is well known that e.g. propylene can be used as a comonomer or as a chain transfer
agent (CTA), or both, whereby it can contribute to the total amount of the vinyl groups.
Herein, when a compound which can also act as comonomer, such as propylene, is used
as CTA for providing double bonds, then said copolymerisable comonomer is not calculated
to the comonomer content.
[0046] Typically, and preferably in wire and cable (W&C) applications, the density of the
polyolefin (A), preferably of the unsaturated LDPE homopolymer or copolymer, is higher
than 860 kg/m
3. Preferably the density of the polyolefin (A), preferably of the unsaturated LDPE
homopolymer or copolymer, is not higher than 960 kg/m
3, and preferably is from 900 to 945 kg/m
3. The MFR
2 (2.16 kg, 190°C) of the polyolefin (A), preferably of the unsaturated LDPE homopolymer
or copolymer, is preferably from 0.01 to 50 g/10min, more preferably from 0.01 to
30.0 g/10, more preferably is from 0.1 to 20 g/10min, and most preferably is from
0.2 to 10 g/10min.
[0047] In most preferred embodiment of the polymer composition the polyolefin (A) is the
unsaturated LDPE polymer as defined above, preferably an unsaturated LDPE copolymer
of ethylene with at least a polyunsaturated comonomer and the antioxidant is 2,4-bis(octylthiomethyl)-6-methylphenol
(CAS number 110553-27-0).
[0048] Accordingly, the polyolefin (A) of the invention is preferably a LDPE polymer, preferably
an unsaturated LDPE copolymer of ethylene with at least a polyunsaturated comonomer,
which is preferably produced at high pressure by free radical initiated polymerisation
(referred to as high pressure (HP) radical polymerization). The HP reactor can be
e.g. a well known tubular or autoclave reactor or a combination thereof, preferably
a tubular reactor. The high pressure (HP) polymerisation and the adjustment of process
conditions for further tailoring the other properties of the polyolefin depending
on the desired end application are well known and described in the literature, and
can readily be used by a skilled person. Suitable polymerisation temperatures range
up to 400°C, preferably from 80 to 350°C and pressure from 70 MPa, preferably 100
to 400 MPa, more preferably from 100 to 350 MPa. Pressure can be measured at least
after compression stage and/or after the tubular reactor. Temperature can be measured
at several points during all steps.
[0049] After the separation the obtained LDPE is typically in a form of a polymer melt which
is normally mixed and pelletized in a pelletising section, such as pelletising extruder,
arranged in connection to the HP reactor system. Optionally, additive(s), such as
antioxidant(s), can be added in this mixer in a known manner.
[0051] As mentioned above, the vinyl group content of the unsaturated LDPE copolymer of
ethylene with at least a polyunsaturated comonomer can be adjusted e.g. with any of
the following means: by polymerising the ethylene e.g. in the presence of one or more
polyunsaturated comonomer(s), optionally in the presence of a chain transfer agent(s),
using the desired feed ratio between monomer, preferably ethylene, and polyunsaturated
comonomer and optionally of the chain transfer agent, as known for a skilled person.
I.a.
WO 9308222 describes a high pressure radical polymerisation of ethylene with polyunsaturated
monomers. As a result the unsaturation can be uniformly distributed along the polymer
chain in random copolymerisation manner. Also e.g.
WO 9635732 describes high pressure radical polymerisation of ethylene and a certain type of
polyunsaturated α,ω-divinylsiloxanes.
[0052] The polymer composition of the invention is highly suitable for wire and cable applications,
particularly as a layer material of a cable.
[0053] The invention also provides a cable which is selected from
- a cable (A) comprising a conductor surrounded by at least one layer comprising, preferably
consisting of, the polymer composition which comprises
- a polyolefin (A) and
- an antioxidant selected from a phenol which bears two substituents both containing
a sulphur atom and optionally further substituent(s), and
wherein the polymer composition comprises vinyl groups/1000 carbon atoms in an amount
of 0.15/1000 carbon atoms or more, when determined according to "double bond content"
described below under Determination methods, as defined above or in claims; or
- a cable (B) comprising a conductor surrounded by an inner semiconductive layer, an
insulating layer and an outer semiconductive layer, wherein at least the insulation
layer comprises, preferably consists of, the polymer composition which comprises
- a polyolefin (A) and
- an antioxidant selected from a phenol which bears two substituents both containing
a sulphur atom and optionally further substituent(s), and
wherein the polymer composition comprises vinyl groups/1000 carbon atoms in an amount
of 0.15/1000 carbon atoms or more, when determined according to "double bond content"
described below under Determination methods, as defined above or in claims.
[0054] The term "conductor" means herein above and below that the conductor comprises one
or more wires. Moreover, the cable may comprise one or more such conductors. Preferably
the conductor is an electrical conductor and comprises one or more metal wires.
[0055] "Semiconductive layer" means herein that said layer comprises a conductive filler,
such as carbon black and has a volume resistivity of 100 000 Ω-cm or below when measured
at 23°C or 90°C, or, when measured according to ISO 3915 (1981) using a plaque, has
a volume resistivity of 100 Ω-cm or below at 23°C, or of 1000 Ω-cm or below at 90°C.
[0056] The cable of the invention is preferably a power cable selected from a LV, MV, HV
or extra high voltage (EHV) cable. The cable (A) is preferably a LV or a MV cable.
The cable (B) is preferably a power cable operating at any voltages, e.g. is a MV
cable, a HV cable or EHV cable.
[0057] The outer semiconductive layer of the cable (B) can be bonded or strippable, i.e.
peeleable, which terms have a well known meaning.
[0058] Preferred cable comprises a layer of a crosslinkable polymer composition.
[0059] The most preferred cable is the cable (B), which is a power cable and preferably
crosslinkable.
[0060] Insulating layers for medium or high voltage power cables generally have a thickness
of at least 2 mm, typically of at least 2.3 mm, and the thickness increases with increasing
voltage the cable is designed for.
[0061] As well known the cable can optionally comprise further layers, e.g. layers surrounding
the insulation layer or, if present, the outer semiconductive layers, such as screen(s),
a jacketing layer(s), other protective layer(s) or any combinations thereof.
[0062] As already mentioned the cable of the invention is preferably crosslinkable. Accordingly,
further preferably the cable is preferably a crosslinked cable (A), wherein at least
one layer comprises a crosslinkable polymer composition of the invention which is
crosslinked before the subsequent end use; or, and preferably, a crosslinked cable
(B), wherein at least the insulation layer comprises crosslinkable polymer composition
of the invention which is crosslinked before the subsequent end use.
[0063] The invention further provides a process for producing
- (i) a cable (A) as defined above, wherein the process comprises the steps of
(a1) providing and mixing, preferably meltmixing in an extruder, a polymer composition
which comprises vinyl groups/1000 carbon atoms in an amount of 0.15/1000 carbon atoms
or more, when determined according to "double bond content" described below under
Determination methods, as defined above or in claims;
(b1) applying the meltmix of the polymer composition obtained from step (a1), preferably
by (co)extrusion, on a conductor to form at least one layer of the cable (A); and
(c1) optionally crosslinking the obtained at least one layer in the presence of the
crosslinking agent; or
- (ii) a cable (B) as defined above comprising a conductor surrounded by an inner semiconductive
layer, an insulation layer, and an outer semiconductive layer, in that order, wherein
the process comprises the steps of (a1)
- providing and mixing, preferably meltmixing in an extruder, a first semiconductive
composition comprising a polymer, a conductive filler and optionally further component(s)
for the inner semiconductive layer,
- providing and mixing, preferably meltmixing in an extruder, a polymer composition
for the insulation layer,
- providing and mixing, preferably meltmixing in an extruder, a second semiconductive
composition comprising a polymer, a conductive filler and optionally further component(s)
for the outer semiconductive layer; (b1)
- applying on a conductor, preferably by coextrusion,
- the meltmix of the first semiconductive composition obtained from step (a1) to form
the inner semiconductive layer,
- the meltmix of polymer composition obtained from step (a1) to form the insulation
layer, and
- the meltmix of the second semiconductive composition obtained from step (a1) to form
the outer semiconductive layer,
wherein at least the polymer composition of the obtained insulation layer comprises,
preferably consists of, a polymer composition which comprises vinyl groups/1000 carbon
atoms in an amount of 0.15/1000 carbon atoms or more, when determined according to
"double bond content" described below under Determination methods, as defined above
or in claims; and
(c1) optionally crosslinking the obtained insulation layer, optionally the obtained
inner semiconductive layer and optionally the outer semiconductive layer in the presence
of a crosslinking agent.
[0064] The term "(co)extrusion" means herein that in case of two or more layers, said layers
can be extruded in separate steps, or at least two or all of said layers can be coextruded
in a same extrusion step, as well known in the art. The term "(co)extrusion" means
herein also that all or part of the layer(s) are formed simultaneously using one or
more extrusion heads.
[0065] As well known a meltmix of the polymer composition or component(s) thereof, is applied
to form a layer. Meltmixing means mixing above the melting point of at least the major
polymer component(s) of the obtained mixture and is carried out for example, without
limiting to, in a temperature of at least 10°C, or at least 15°C, above the melting
or softening point of polymer component(s). The meltmixing can be carried out in the
cable extruder. The mixing step (a1) may comprise a separate mixing step in a separate
mixer, e.g. kneader, arranged in connection and preceding the cable extruder of the
cable production line. Mixing in the preceding separate mixer can be carried out by
mixing with or without external heating (heating with an external source) of the component(s).
[0066] The polymer composition can be produced before or during the cable production process.
Moreover the polymer composition(s) of the layer(s) can each independently comprise
part or all of the components of the final composition, before providing to the (melt)mixing
step (a1) of the cable production process. Then the remaining component(s) are provided
prior to or during the cable formation.
[0067] Accordingly, the polymer composition is provided to step (a1) already in a form of
a blend which is produced beforehand by mixing the polyolefin (A) and the antioxidant
together; or the polyolefin (A) and the antioxidant are provided separately to step
(a1) and mixed together during the mixing step (a1) to form the polymer composition.
[0068] When the polymer composition is provided to step (a1) as a blend, then the antioxidant
can be mixed with the polyolefin (A), e.g. by meltmixing, and the obtained meltmix
is pelletized to pellets for use in cable production. Pellets mean herein generally
any polymer product which is formed from reactor-made polymer (obtained directly from
the reactor) by post-reactor modification to a solid polymer particles. Pellets can
be of any size and shape. The obtained pellets are then used in step (a1) of the cable
production process.
[0069] Alternatively, the polyolefin (A) and the antioxidant can be provided separately
to the cable production line.
[0070] In the preferred cable production process the antioxidant is added to polyolefin
(A) in form of a liquid to form the blend which is then provided to step (a1) or the
antioxidant is added separately in liquid form to polyolefin (A) during the mixing
step (a1) Preferably the antioxidant product is in liquid state e.g. at room temperature.
If not, then it can be dissolved in a solvent or melted before the addition. The addition
of the antioxidant contributes beneficially to the desired improved WTR property.
[0071] All or part of the optional other component(s), such as further polymer component(s)
or additive(s) can be present in the polymer composition before providing to the cable
preparation process or can be added, e.g by the cable producer, during the cable production
process.
[0072] If, and preferably, the polymer composition is crosslinked after cable formation,
then the crosslinking agent is preferably a peroxide, which can be mixed with the
components of the polymer composition before or during step (a1). It is preferred
that the peroxide is added in form of a liquid to the polyolefin (A). Still more preferably,
the crosslinking agent, preferably peroxide, is impregnated to the solid polymer pellets
of the polymer composition. The obtained pellets are then provided to the cable production
step. Due to unsaturation the amount of peroxide can be decreased. Thus the amount
of the preferable liquid peroxide can be decreased and the antioxidant can also be
added in liquid form, however still maintaining a beneficial low amount of added liquids
to the polyolefin (A). As a result good quality and excellent WTR properties are obtained.
[0073] Most preferably, the polymer composition of the invention is provided to the step
(a1) of the cable production process in a suitable product form, such as a pellet
product.
[0074] As mentioned, the polymer composition is preferably crosslinkable and preferably
the pellets of the polymer composition comprise also the peroxide before providing
to the cable production line.
[0075] In the preferred cable production process the obtained cable is crosslinked in step
(c1), more preferably the at least one layer of formed cable (A) or at least the insulation
layer of the formed cable (B) is crosslinked in step (c1) in the presence of a free
radical forming agent, preferably peroxide.
[0076] As above, the preferred cable production process embodiment of the invention is for
producing a power cable (B).
[0077] Most preferably, the cable, preferably the power cable (B), of the invention is crosslinked
after the formation of cable layers. In this preferred cable production embodiment
a power cable (B) is produced, wherein at least the insulation layer of cable (B)
comprises the polymer composition as defined above or in claims and wherein the insulation
layer, optionally the inner semiconductive layer and optionally the outer semiconductive
layer of the cable (B) is crosslinked in the crosslinking step via radical reaction,
preferably in the presence of a crosslinking agent which is preferably peroxide.
[0078] In above crosslinking process step (c1) of the invention crosslinking conditions
can vary depending i.a. on the used crosslinking method, and cable size. The crosslinking
of the invention is effected e.g. in a known manner preferably in an elevated temperature.
A skilled person can choose the suitable crosslinking conditions e.g. for crosslinking
via radical reaction or via hydrolysable silane groups. As non-limiting example of
a suitable crosslinking temperature range, e.g. at least 150°C and typically not higher
than 360°C.
Determination methods
[0079] Unless otherwise stated the below determination methods were used to determine the
properties defined generally in the description part and claims and in the experimental
part.
[0081] Melt Flow Rate: The melt flow rate (MFR) is determined according to ISO 1133 and is indicated in
g/10 min. The MFR is an indication of the flowability, and hence the processability,
of the polymer. The higher the melt flow rate, the lower the viscosity of the polymer.
The MFR is determined at 190°C for polyethylenes and may be determined at different
loadings such as 2.16 kg (MFR
2) or 21.6 kg (MFR
21). The MFR is determined at 230°C for polypropylenes.
[0082] Density: Low density polyethylene (LDPE): The density is measured according to ISO 1183-2.
The sample preparation was executed according to ISO 1872-2 Table 3 Q (compression
moulding).
Comonomer contents
[0083]
- a) Comonomer content in random copolymer of polypropylene:
Quantitative Fourier transform infrared (FTIR) spectroscopy was used to quantify the
amount of comonomer. Calibration was achieved by correlation to comonomer contents
determined by quantitative nuclear magnetic resonance (NMR) spectroscopy.
The calibration procedure based on results obtained from quantitative 13C-NMR spectroscopy was undertaken in the conventional manner well documented in the
literature.
The amount of comonomer (N) was determined as weight percent (wt%) via:
wherein A is the maximum absorbance defined of the comonomer band, R the maximum absorbance
defined as peak height of the reference peak and with k1 and k2 the linear constants
obtained by calibration. The band used for ethylene content quantification is selected
depending if the ethylene content is random (730 cm-1) or block-like (as in heterophasic PP copolymer) (720 cm-1). The absorbance at 4324 cm-1 was used as a reference band.
- b) Quantification of alpha-olefin content in linear low density polyethylenes and
low density polyethylenes by NMR spectroscopy:
The comonomer content was determined by quantitative 13C nuclear magnetic resonance
(NMR) spectroscopy after basic assignment (J. Randall JMS - Rev. Macromol. Chem. Phys.,
C29(2&3), 201-317 (1989). Experimental parameters were adjusted to ensure measurement
of quantitative spectra for this specific task.
[0084] Specifically solution-state NMR spectroscopy was employed using a Bruker AvanceIII
400 spectrometer. Homogeneous samples were prepared by dissolving approximately 0.200
g of polymer in 2.5 ml of deuterated-tetrachloroethene in 10 mm sample tubes utilising
a heat block and rotating tube oven at 140 C. Proton decoupled 13C single pulse NMR
spectra with NOE (powergated) were recorded using the following acquisition parameters:
a flip-angle of 90 degrees, 4 dummy scans, 4096 transients an acquisition time of
1.6s, a spectral width of 20kHz, a temperature of 125 C, a bilevel WALTZ proton decoupling
scheme and a relaxation delay of 3.0 s. The resulting FID was processed using the
following processing parameters: zero-filling to 32k data points and apodisation using
a gaussian window function; automatic zeroth and first order phase correction and
automatic baseline correction using a fifth order polynomial restricted to the region
of interest.
[0085] Quantities were calculated using simple corrected ratios of the signal integrals
of representative sites based upon methods well known in the art.
c) Comonomer content of polar comonomers in linear low density polyethylene
(1) Polymers containing > 6 wt.% polar comonomer units
[0086] Comonomer content (wt%) was determined in a known manner based on Fourier transform
infrared spectroscopy (FTIR) determination calibrated with quantitative nuclear magnetic
resonance (NMR) spectroscopy. Below is exemplified the determination of the polar
comonomer content of ethylene ethyl acrylate, ethylene butyl acrylate and ethylene
methyl acrylate. Film samples of the polymers were prepared for the FTIR measurement:
0.5-0.7 mm thickness was used for ethylene butyl acrylate and ethylene ethyl acrylate
and 0.10 mm film thickness for ethylene methyl acrylate in amount of >6wt%. Films
were pressed using a Specac film press at 150°C, approximately at 5 tons, 1-2 minutes,
and then cooled with cold water in a not controlled manner. The accurate thickness
of the obtained film samples was measured.
[0087] After the analysis with FTIR, base lines in absorbance mode were drawn for the peaks
to be analysed. The absorbance peak for the comonomer was normalised with the absorbance
peak of polyethylene (e.g. the peak height for butyl acrylate or ethyl acrylate at
3450 cm
-1 was divided with the peak height of polyethylene at 2020 cm
-1). The NMR spectroscopy calibration procedure was undertaken in the conventional manner
which is well documented in the literature, explained below.
[0088] For the determination of the content of methyl acrylate a 0.10 mm thick film sample
was prepared. After the analysis the maximum absorbance for the peak for the methylacrylate
at 3455 cm
-1 was subtracted with the absorbance value for the base line at 2475 cm
-1 (A
methylacrylate - A
2475). Then the maximum absorbance peak for the polyethylene peak at 2660 cm
-1 was subtracted with the absorbance value for the base line at 2475 cm
-1 (A
2660 -A
2475). The ratio between (A
methylacrylate-A
2475) and (A
2660-A
2475) was then calculated in the conventional manner which is well documented in the literature.
[0089] The weight-% can be converted to mol-% by calculation. It is well documented in the
literature.
[0090] Quantification of copolymer content in polymers by NMR spectroscopy
[0091] The comonomer content was determined by quantitative nuclear magnetic resonance (NMR)
spectroscopy after basic assignment (e.g. "NMR Spectra of Polymers and Polymer Additives",
A. J. Brandolini and D. D. Hills, 2000, Marcel Dekker, Inc. New York). Experimental
parameters were adjusted to ensure measurement of quantitative spectra for this specific
task (e.g "
200 and More NMR Experiments: A Practical Course", S. Berger and S. Braun, 2004, Wiley-VCH,
Weinheim). Quantities were calculated using simple corrected ratios of the signal integrals
of representative sites in a manner known in the art.
(2) Polymers containing 6 wt.% or less polar comonomer units
[0092] Comonomer content (wt.%) was determined in a known manner based on Fourier transform
infrared spectroscopy (FTIR) determination calibrated with quantitative nuclear magnetic
resonance (NMR) spectroscopy. Below is exemplified the determination of the polar
comonomer content of ethylene butyl acrylate and ethylene methyl acrylate. For the
FT-IR measurement a film samples of 0.05 to 0.12 mm thickness were prepared as described
above under method 1). The accurate thickness of the obtained film samples was measured.
[0093] After the analysis with FT-IR base lines in absorbance mode were drawn for the peaks
to be analysed. The maximum absorbance for the peak for the comonomer (e.g. for methylacrylate
at 1164 cm
-1 and butylacrylate at 1165 cm
-1) was subtracted with the absorbance value for the base line at 1850 cm
-1 (A
polar comonomer - A
1850). Then the maximum absorbance peak for polyethylene peak at 2660 cm
-1 was subtracted with the absorbance value for the base line at 1850 cm
-1 (A2660 - A
1850). The ratio between (A
comonomer-A
1850) and (A
2660-A
1850) was then calculated. The NMR spectroscopy calibration procedure was undertaken in
the conventional manner which is well documented in the literature, as described above
under method 1).
[0094] The weight-% can be converted to mol-% by calculation. It is well documented in the
literature.
[0095] Below is exemplified how polar comonomer content obtained from the above method (1)
or (2), depending on the amount thereof, can be converted to micromol or mmol per
g polar comonomer as used in the definitions in the text and claims: The millimoles
(mmol) and the micro mole calculations have been done as described below.
[0096] For example, if 1 g of the poly(ethylene-co-butylacrylate) polymer, which contains
20 wt% butylacrylate, then this material contains 0.20/M
butylacrylate (128 g/mol) = 1.56 x 10
-3 mol. (=1563 micromoles).
[0097] The content of polar comonomer units in the polar copolymer C
polar comonomer is expressed in mmol/g (copolymer). For example, a polar poly(ethylene-co-butylacrylate)
polymer which contains 20 wt.% butyl acrylate comonomer units has a C
polar comonomer of 1.56 mmol/g.
[0098] The used molecular weights are: M
butylacrylate = 128 g/mole, M
ethylacrylate = 100 g/mole, M
methylacrylate =86 g/mole).
Carbon-Carbon Double bond content:
A) Quantification of the amount of carbon-carbon double bonds by IR
spectroscopy
[0099] Quantitative infrared (IR) spectroscopy was used to quantify the amount of carbon-carbon
double bonds (C=C). Calibration was achieved by prior determination of the molar extinction
coefficient of the C=C functional groups in representative low molecular weight model
compounds of known structure.
[0100] The amount of each of these groups (N) was determined as number of carbon-carbon
double bonds per thousand total carbon atoms (C=C/1000C) via:
were A is the maximum absorbance defined as peak height, E the molar extinction coefficient
of the group in question (l·mol
-1·mm
-1), L the film thickness (mm) and D the density of the material (g·cm
-1).
[0101] The total amount of C=C bonds per thousand total carbon atoms can be calculated through
summation of N for the individual C=C containing components.
[0102] For polyethylene samples solid-state infrared spectra were recorded using a FTIR
spectrometer (Perkin Elmer 2000) on compression moulded thin (0.5-1.0 mm) films at
a resolution of 4 cm
-1 and analysed in absorption mode.
1) Polymer compositions comprising polyethylene homopolymers and copolymers, except
polyethylenes copolymers with > 0.4 wt% polar comonomer
[0103] For polyethylenes three types of C=C containing functional groups were quantified,
each with a characteristic absorption and each calibrated to a different model compound
resulting in individual extinction coefficients:
- vinyl (R-CH=CH2) via 910 cm-1 based on 1-decene [dec-1-ene] giving E = 13.13 1·mol-1·mm-1
- vinylidene (RR'C=CH2) via 888 cm-1 based on 2-methyl-1-heptene [2-methyhept-1-ene] giving E = 18.24 1·mol-1-mm-1
- trans-vinylene (R-CH=CH-R') via 965 cm-1 based on trans-4-decene [(E)-dec-4-ene] giving E = 15.14 1·mol-1·mm-1
[0104] For polyethylene homopolymers or copolymers with < 0.4 wt% of polar comonomer linear
baseline correction was applied between approximately 980 and 840 cm
-1.
2) Polymer compositions comprising polyethylene copolymers with > 0.4 wt% polar comonomer
[0105] For polyethylene copolymers with > 0.4 wt% of polar comonomer two types of C=C containing
functional groups were quantified, each with a characteristic absorption and each
calibrated to a different model compound resulting in individual extinction coefficients:
- vinyl (R-CH=CH2) via 910 cm-1 based on 1-decene [dec-1-ene] giving E = 13.13 1·mol-1·mm-1
- vinylidene (RR'C=CH2) via 888 cm-1 based on 2-methyl-1-heptene [2-methyl-hept-1-ene] giving E = 18.24 1-mol-1·mm-1
EBA:
[0106] For poly(ethylene-co-butylacrylate) (EBA) systems linear baseline correction was
applied between approximately 920 and 870 cm
-1.
EMA:
[0107] For poly(ethylene-co-methylacrylate) (EMA) systems linear baseline correction was
applied between approximately 930 and 870 cm
-1.
3) Polymer compositions comprising unsaturated low molecular weight molecules
[0108] For systems containing low molecular weight C=C containing species direct calibration
using the molar extinction coefficient of the C=C absorption in the low molecular
weight species itself was undertaken.
B) Quantification of molar extinction coefficients by IR spectroscopy
[0109] The molar extinction coefficients were determined according to the procedure given
in ASTM D3124-98 and ASTM D6248-98. Solution-state infrared spectra were recorded
using a FTIR spectrometer (Perkin Elmer 2000) equipped with a 0.1 mm path length liquid
cell at a resolution of 4 cm
-1.
[0110] The molar extinction coefficient (E) was determined as 1·mol
-1·mm
-1 via: E = A / (C x L)
where A is the maximum absorbance defined as peak height, C the concentration (mol·l
-1) and L the cell thickness (mm).
[0111] At least three 0.18 mol·l
-1 solutions in carbondisulphide (CS
2) were used and the mean value of the molar extinction coefficient determined.
Water Treeing test Method:
[0112] The samples were prepared according to ASTM D6097-97a, which is a water treeing test
method. Plaques were prepared from pellets containing both the antioxidant and the
peroxide (the antioxidant and the peroxide are added in form of a liquid and separately
to polymer) by preheating for 5 min and compression moulding for 5 min at 120 °C,
20 bars and 5 min at 120 °C at 200 bars followed by cooling to room temperature with
a cooling rate of 14 °C/min. From this plaque 12 circular disc shaped specimens with
a needle deformation were prepared and crosslinked in a mould especially designed
for this type of water treeing test. For this second preparation step the plaque was
preheated for 10 min and thereafter moulded for 10 min at 120 °C at 20 bars and then
crosslinked for 10 min at 180°C at 200 bars followed by cooling to room temperature
by a cooling rate of 14 °C. The resulting disc samples had a thickness or around 6.4
mm, a diameter of around 25.4 mm and a needle depth of around 3.2 mm. These discs
were degassed for 168 h at 80 °C under vacuum to remove by-product originating from
the peroxide. The specimens were fixed in an insulating box which was placed in a
water bath filled with 0.01 M sodium chloride electrolyte and water trees were grown
at 5 kV and 1000 Hz for 30 days at room temperature. After completed ageing the test
specimens were sliced with a microtome to 150-300 µm thick samples which were stained
with methylene blue for approximately 1 h. The length and the width of the water trees
were determined by a visual inspection in a light microscope.
Experimental Part
[0113] The following components were used in the inventive examples of the polymer composition
of the invention given below.
LDPE 1: Ethylene-1,7-octadiene copolymer
[0114] Purified ethylene was liquefied by compression and cooling to a pressure of 90 bars
and a temperature of -30 °C and split up into to two equal streams of roughly 14 tons/hour
each. The CTA (methyl ethyl ketone (MEK)), air and a commercial peroxide radical initiator
dissolved in a solvent were added to the two liquid ethylene streams in individual
amounts. 1,7-octadiene as a comonomer was added to the reactor in amount of 190 kg/h.
The two mixtures were separately pumped through an array of 4 intensifiers to reach
pressures of 2200-2300 bars and exit temperatures of around 40 °C. These two streams
were respectively fed to the front (zone 1) (50%) and side (zone 2) (50%) of a split-feed
two-zone tubular reactor. The inner diameters and lengths of the two reactor zones
were 32 mm and 200 m for zone 1 and 38 mm and 400 m for zone 2. MEK was added in amounts
of 115 kg/h to the front stream to maintain a MFR
2 of around 2.1 g/10 min. The front feed stream was passed through a heating section
to reach a temperature sufficient for the exothermal polymerization reaction to start.
The reaction reached peak temperatures were 253 °C and 290 °C in the first and second
zones, respectively. The side feed stream cooled the reaction to an initiation temperature
of the second zone of 165 °C. Air and peroxide solution was added to the two streams
in enough amounts to reach the target peak temperatures. The reaction mixture was
depressurized by product valve, cooled and polymer was separated from unreacted gas.
[0115] The obtained LDPE1 had Vinyl groups in amount of 0.82/1000 C and MFR
2 = 2.1 g/10 min
LDPE 2: Homopolymer of ethylene (Comparative polymer)
[0116] Purified ethylene was liquefied by compression and cooling to a pressure of 90 bars
and a temperature of -30°C and split up into to two equal streams of roughly 14 tons/hour
each. The CTA (methyl ethyl ketone, MEK), air and a commercial peroxide radical initiator
dissolved in a solvent were added to the two liquid ethylene streams in individual
amounts. The two mixtures were separately pumped through an array of 4 intensifiers
to reach pressures of 2100-2300 bars and exit temperatures of around 40°C. These two
streams were respectively fed to the front (zone 1) (50%) and side (zone 2) (50%)
of a split-feed two-zone tubular reactor. The inner diameters and lengths of the two
reactor zones were 32 mm and 200 m for zone 1 and 38 mm and 400 m for zone 2. MEK
was added in amounts of around 216 kg/h to the front stream to maintain a MFR
2 of around 2 g/10 min. The front feed stream was passed through a heating section
to reach a temperature sufficient for the exothermal polymerization reaction to start.
The reaction reached peak temperatures were around 250 °C and 318 °C in the first
and second zones, respectively.. The side feed stream cooled the reaction to an initiation
temperature of the second zone of 165-170°C. Air and peroxide solution was added to
the two streams in enough amounts to reach the target peak temperatures. The reaction
mixture was depressurized by product valve, cooled and polymer was separated from
unreacted gas.
[0117] The obtained LDPE2 had Vinyl groups in amount of 0.11/1000 C and MFR
2 = 2.0 g/10 min.
[0118] Antioxidant (AO): 2,4-bis(octylthiomethyl)-6-methylphenol (CAS number 110553-27-0) which is commercially
available.
[0119] Peroxide (POX): dicumylperoxide (CAS number 80-43-3)
[0120] The test polymer compositions are given in table 1 which also shows the results of
the WTR determination.
Table 1: The given (wt% based on the total amount of the polymer composition:
|
LDPE1
(wt%) |
LDPE2
(wt%) |
AO
(wt%) |
POX
(wt%) |
Water tree length, (mm) |
Water tree width (mm) |
Inventive comp. 1 |
97.69 |
|
0.21 |
0.95 |
0.50 |
0.76 |
Comparative comp. 1 |
|
98.84 |
0.21 |
2.1 |
0.73 |
1.07 |
Vinyl groups/1000C |
0.82/1000C |
0.11/1000C |
|
|
|
|
[0121] The results show the improved WTR properties, both in the length and width of the
water tree, of the inventive composition 1 which contains the inventive combination
of vinyl groups together with the antioxidant compared to comparative composition.
[0122] Moreover, the inventive composition 1 and the comparative composition had the similar
crosslinking level, although the amount of the added peroxide of the inventive composition
1 could be kept lower. And still the inventive composition 1 has still improved WTR
property is achieved.
1. A polymer composition comprising
- a polyolefin (A) and
- an antioxidant selected from a phenol which bears two substituents both containing
a sulphur atom and optionally further substituent(s), and
wherein the polymer composition comprises vinyl groups/1000 carbon atoms in an amount
of 0.15/1000 carbon atoms or more, when determined according to "double bond content"
described above under Determination methods.
2. The polymer composition according to claim 1, wherein the antioxidant is bis[(C1-C12)alkylthio(C1-C12)alkyl]phenol
which optionally bears further substituent(s),
3. The polymer composition according to any of the previous claims, wherein the antioxdant
is 2,4-bis(octylthiomethyl)-6-methylphenol (CAS number 110553-27-0).
4. The polymer composition according to any of the previous claims, wherein the polyolefin
(A) is unsaturated and comprises vinyl groups/1000 carbon atoms in an amount of 0.15/1000
carbon atoms or more, preferably in an amount of 0.20/1000 carbon atoms or more, and
preferably the amount of vinyl groups/1000 carbon atoms is less than 4.0/1000 carbon
atoms.
5. The polymer composition according to any of the previous claims, wherein the unsaturated
polyolefin (A) is an unsaturated polyethylene.
6. The polymer composition according to any of the previous claims, wherein the unsaturated
polyolefin (A) is an unsaturated polyethylene produced in a high pressure polymerisation
process, preferably an unsaturated low density homopolymer of ethylene (LDPE homopolymer)
or an unsaturated copolymer of ethylene with at least one comonomer (LDPE copolymer),
more preferably an unsaturated LDPE copolymer of ethylene with at least a polyunsaturated
comonomer, preferably with at least a polyunsaturated comonomer consisting of a straight
carbon chain with at least 8 carbon atoms and at least 4 carbons between the non-conjugated
double bonds, of which at least one is terminal.
7. The polymer composition according to any of the previous claims, wherein the unsaturated
polyolefin (A) is an unsaturated LDPE copolymer of ethylene with at least a polyunsaturated
comonomer which is a diene which is selected from C8- to C14-non-conjugated diene or mixtures thereof, more preferably from 1,7-octadiene, 1,9-decadiene,
1,11-dodecadiene, 1,13-tetradecadiene, 7-methyl-1,6-octadiene, 9-methyl-1,8-decadiene,
or mixtures thereof, more preferably from 1,7-octadiene, 1,9-decadiene, 1,11-dodecadiene,
1,13-tetradecadiene, or any mixture thereof.
8. The polymer composition according to any of the previous claims, wherein the polymer
composition further comprises a crosslinking agent.
9. The polymer composition according to any of the previous claims, wherein the crosslinking
agent is a free radical generating agent, preferably peroxide.
10. A cable which is selected from
- a cable (A) comprising a conductor surrounded by at least one layer comprising the
polymer composition as defined in any of the preceding claims 1 to 9; or
- a cable (B), preferably a power cable (B), comprising a conductor surrounded by
an inner semiconductive layer, an insulating layer and an outer semiconductive layer,
wherein at least the insulation layer, comprises the polymer composition as defined
in any of the preceding claims 1 to 9.
11. A process for producing
(i) a cable (A) according to claim 10, wherein the process comprises the steps of
(a1) providing and mixing, preferably meltmixing in an extruder, a polymer composition
as defined in any of the preceding claims 1 to 9;
(b1) applying the meltmix of the polymer composition obtained from step (a1), preferably
by (co)extrusion, on a conductor to form at least one layer of the cable (A); and
(c1) optionally crosslinking the obtained at least one layer in the presence of the
crosslinking agent; or
(ii) a cable (B) according to claim 10 comprising a conductor surrounded by an inner
semiconductive layer, an insulation layer, and an outer semiconductive layer, in that
order, wherein the process comprises the steps of
(a1)
- providing and mixing, preferably meltmixing in an extruder, a first semiconductive
composition comprising a polymer, a conductive filler and optionally further component(s)
for the inner semiconductive layer,
- providing and mixing, preferably meltmixing in an extruder, a polymer composition
for the insulation layer,
- providing and mixing, preferably meltmixing in an extruder, a second semiconductive
composition comprising a polymer, a conductive filler and optionally further component(s)
for the outer semiconductive layer;
(b1)
- applying on a conductor, preferably by coextrusion,
- the meltmix of the first semiconductive composition obtained from step (a1) to form
the inner semiconductive layer,
- the meltmix of polymer composition obtained from step (a1) to form the insulation
layer, and
- the meltmix of the second semiconductive composition obtained from step (a1) to
form the outer semiconductive layer,
wherein at least the polymer composition of the obtained insulation layer comprises
a polymer composition as defined in any of the preceding claims 1 to 9; and
(c1) optionally crosslinking the obtained insulation layer, optionally the obtained
inner semiconductive layer and optionally the outer semiconductive layer in the presence
of a crosslinking agent.
12. The process of claim 11, wherein the polymer composition as defined in any of the
preceding claims is provided to step (a1) already in a form of a blend which is produced
beforehand by mixing the polyolefin (A) and the antioxidant together; or the polyolefin
(A) and the antioxidant are provided separately to step (a1) and mixed together during
the mixing step (a1 to form the polymer composition.
13. The process of claim 11 or 12, wherein the antioxidant is added to polyolefin (A)
in form of a liquid to form the blend which is then provided to step (a1); or the
antioxidant is added separately in liquid form to polyolefin (A) during the mixing
step (a1).
14. The process according to any of the previous claims 11 to 13, wherein the at least
one layer of formed cable (A) or at least the insulation layer of the formed cable
(B) is crosslinked in step (c1) in the presence of a free radical forming agent.
15. The process according to any of the previous claims 11 to 14, wherein the crosslinking
agent, preferably peroxide, is added to polyolefin (A) in form of a liquid when producing
the blend of the polyolefin (A) and the antioxidant and the blend is then provided
to step (a1); or the crosslinking agent, preferably peroxide, is added separately
in liquid form to polyolefin (A) during the mixing step (a1).